1. Field of the Invention
This invention relates to the construction of a reinforced concrete structure. More specifically, the invention relates to a method of manufacturing an arched or barrel vaulted concrete structure such as a building or pipeline by applying a cementitious composition on a highly pressurized, inflatable form which is confined by steel straps and/or wire mesh.
2. Description of the Prior Art
The basic concept of manufacturing a thin walled reinforced fiber containing structure such as a building, pipe or the like is generally a well established art. However, when attempting to employ this art in combination with removable and reuseable inflatable molds or forms, the available alternative methods of construction are plagued with problems that severely limit the commercial acceptance of such techniques. Yet in principle, a reliable and safe construction technique using an inflatable mold and fiber containing cement should lead to an extremely cost effective and structurally acceptable product.
Among the problems encountered when fabricating concrete on an inflatable form has been the tendency for the form to be displaced and distorted when heavy loads of cementitious composition are applied to the form. These problems have been further complicated by the tendency of the inflatable form to vibrate and transmit relative movement during application and setting of the concrete. Various methods and techniques have been suggested and employed to compensate for such problems, particularly when building large scale cement shell structures and the like. For example, in U.S. Pat. No. 2,388,701 a "workdown" procedure for cementing a hemispherical open bottomed, inflatable form mounted on a circular foundation is disclosed. This "workdown" procedure involves the cement to the top third of the form first, thus creating a compression on the rest of the form. However, it is recognized in this patent that such a procedure induces significant distortion of the sidewalls of the dome form and resulting structure. This distortion problem is again acknowledged in U.S. Pat. No. 2,892,239 and compensated for by the use of a series of horizontal reinforced bands placed at the base of the domed top resting on vertically straight sidewalls of a circular structure. The straight sidewalls are also reinforced by bands of wire mesh wrapped around the inflatable form and again the workdown procedure is viewed as critical to create the desired structural stress
Other techniques have been employed in relatively thick walled cement structure using inflatable forms such as suggested in U.S. Pat. No. 2,624,931 wherein a staged segmented casting process is employed including an intermediate pouring of the top section midway through the construction. U.S. Pat. No. 3,390,211 discloses a method of making a nuclear reactor vessel from prestressed elements and concrete wall segments which are fabricated to each other with the use of an inflatable form. In fact, when attempting to build thin walled, large scale cement structures, the problems of form distortion and movement during application and the vibration/wave motion during application and setting have resulted in extraordinary measures being employed during fabrication. For example, in U.S. Pat. Nos. 3,462,521 and 4,170,093 a method of casting the cement on the deflated form and then inflating the form and unset cement is proposed. It has also been proposed to work from within an inflated form by applying the cement to the inner surface of the form. In U.S. Pat. No. 3,619,432 the form is stabilized by the presence of a network of wire ropes suspended from the inner layer of the inflated form to which the cement is to be applied. And, in U.S. Pat. No. 4,155,967 a method of initially coating the interior of the inflated form with a sprayed on layer of insulating foam or the like and letting it set up is used prior to the application of the cement, while in U.S. Pat. No. 4,002,707 a plurality of water filled bladders stacked on each other is used to make the form.
The problem of form distortion when using an inflatable form to manufacture an arched or barrel vaulted concrete building is of particular importance and even critical when the building is intended to support an overburden such as in earthen shelters in that the structural strength of the arch is dependent on preserving the geometry of the arch. The significance of the distortion problem has been recognized in the prior art and has led to the suggestion of extraordinary remedies. For example, see U.S. Pat. No. 3,139,464 wherein triangular cross-sectional trusses are positioned along the arched roof to contribute hoop strength to the structure and inflatable form. In this patent, the two step cement pouring process is employed wherein the triangular hoop trusses are initially poured and the concrete is allowed to set up before the region between the trusses are poured. As such, the first step produces a steel reinforced arched truss system that then stabilizes the inflatable form and bears the load when the concrete roof is poured during the second step. Such a process requires the fabrication of specialized triangular trusses and the intentional incorporation of these trusses into the final concrete structure. As such, the presence of the trusses, in principle, represents an unnecessary expense. Thus, the need exists for an inexpensive yet reliable method of fabricating the arched or barrel vaulted structure directly onto the inflatable form without the use of unnecessary structural elements and without encountering the form distortion problem. The present invention is viewed as such a process.